New chapters can be created at 2 levels. If you need to add a chapter at
second level for the existed first level chapter, then specify Major chapter
number as the existed first level (under which you need to create a new chapter)
and specify a unique Minor number, which is usually a consecutive number to the
last available second level chapter. If for instance you need to add a new
second level chapter for 8.0 SE Optimization, then while creating a new chapter
specify Major – 8 and Minor – 6 to create chapter 8.6.

If you need to start a new first level chapter, then use Major – 13 and Minor
– 0. This would create a new first level 13.0 (after last available 12). Then
you may add second level chapter associated with 13.0 – like 13.1, 13.2 .. etc.